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    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script>
        var a = 10;
        f1();

        function f1() {
            var b = 9;
            console.log(a); //undefined
            console.log(b); //9
            var a = '123';
        }
        // 通过预解析，相当于执行以下代码：
        // var a;
        // function f1() {
        //     var b;
        //     var a;
        //     b = 9;
        //     console.log(a);
        //     console.log(b);
        //     a = '123';
        // }
        // a = 10;
        // f1();
    </script>
</head>

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</body>

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